To use the words of a fellow Jew, my religion status is "bad jew". I do not keep kosher all year around. I eat shellfish, meats that weren't kill in the kosher way, and I certainly mix meat and dairy. I don't remember the last time I went synagogue. I don't fast during Yom Kippur. I don't even believe in most of the lessons of the old testament [I do most definitely believe in God]. So the question that I am guessing you have is "why do I keep kosher for passover?". I have two answers to the question. First, passover was a special holiday for my family. My mom took great pride in organizing sedars for the first two nights of passover. We had family and friends over to the house to "celebrate the passover here, but next year we hope to celebrate in the land of Israel". Although I am never going to live in Israel, the lines were often times the first two lines that my father spoke in english during the sedar. Its a family tradition. Its a connection to the family lineage. This brings to my second point. My ancestors died for my religion. They moved from country to country, house to house to ensure that their children and grandchildren would grow up in a Jewish household. I owe it to them in some way respect their sacrifice. They ran from Egypt. If I fail on all other days, it is my duty on passover to realize my religion.
You might still ask why I keep such a strict diet (kinyot comes to mind). Well, my family is Ashkenazi. Maintaining no kinyot maybe difficult, it is probably unnecessary in the eyes of God. It is, however, one extra respect to my family tradition.
Oh and one more reason keep kosher: its bloody hard for someone who only eats carbs. I order out pizza out every night.... now I'm down to nightly visits to the Pavilion...
It is in my mind a selfless gesture to my family.